The need for cooling electronic devices such as infrared detectors to cryogenic temperatures is often met by miniature refrigerators operating on the Stirling cycle principle. As is well known, these cryogenic refrigerators or cryocoolers, use a motor driven compressor to impart a cyclical volume variation to a working volume filled with a pressurized refrigeration gas. The pressurized refrigeration gas is forced through the working volume to one end of a sealed cylinder called a cold well. A piston-shaped heat exchanger or regenerator is movably disposed inside the cold well. The regenerator includes passage ways to allow the refrigeration gas to enter and exit the cold well through the regenerator.
The regenerator reciprocates at a 90.degree. phase shift relative to the compressor piston and the refrigeration gas is force to flow through the cold well in alternating directions. The refrigeration gas is thereby forced to flow from a compression space of the compressor through the regenerator passage ways and into the sealed cold well and then back. As the regenerator reciprocates, a warm end of the cold well which directly receives the refrigeration gas from the compressor becomes much warmer than the ambient. In the opposite end of the cold well, called the expansion space or cold end, the refrigeration gas expands and becomes much colder than the ambient. A device to be cooled is thus mounted adjacent to the expansion space, or cold end of the cold well such that thermal energy from the device to be cooled is passed to the refrigeration gas through a wall of the cold well.
It is a typical problem in the design of cryocooler compressor elements to minimize the amount of thermal energy generated by the operation of the compressor and further to avoid passing thermal energy from the compressor components to the refrigeration gas. It is also a problem in the design of cryocooler systems to improve the efficiency of the cryocooler so that the input power required to drive the compressor and regenerator pistons is reduced. This is especially true for cryocooler systems employed in portable hand held camera systems or other portable devices which typically operate under battery power.
It is known that proper selection of the radial clearance as well as reducing friction between a cryocooler compression piston and its mating compression cylinder bore can improve overall system efficiency and reduce thermal energy generated while operating the compressor. The goal of the compressor designer is to provide a uniform radial clearance between the compression piston and the compression cylinder wall. This allows the working gas to flow uniformly through the radial clearance or circumferencial gap surrounding the compression piston during a compression stroke so that a gas film uniformly supports the compression piston within the compression cylinder bore without contact with the cylinder wall. At the same time the pressure drop across the compression piston during a compression stroke of the piston should be minimized. It is therefore advantageous to have as small a radial gap as possible.
Using conventional manufacturing processes of first rough machining the compression piston and cylinder bore, then hardening the mating surfaces, e.g. by heat treating, then grinding and honing or lapping, the mating surfaces to a final dimension, small working clearances in the range of 50-75 micro inches are achievable. There is a general problem with the conventional techniques, however, that accurate geometry of the mating parts, specifically cylindricity of the piston outside diameter and the cylinder bore, is very difficult to achieve. Non-round and or non-cylindrical mating parts cause a non-uniform radial gap between the compressor piston and the cylinder wall which can lead to non-uniform gas pressure in the gap. This can lead to non-uniform loading of the piston against the cylinder wall causing locally increased friction and uneven wear. As a result, excess thermal energy is generated in the compressor and the energy required to drive the compressor is increased. The inability to maintain accurate part geometry by conventional techniques has forced manufacturers to resort to larger radial clearances than are desired.
It is also a problem that lapping and honing are hand operations which are difficult to automate. This results in increased manufacturing costs and cycle times. Another problem with conventional methods is that lapping compound residue can contaminate the cryocooler unit ultimately shortening the life of the unit. It is a further problem that prior art conventional manufacturing techniques are most suitable for use with steel whereas it is more desirable to manufacture compressor elements from aluminum or copper which have a higher thermal conductivity for more readily removing thermal energy from the working gas and the compressor.
It is known to reduced friction between the compressor piston and the mating cylinder wall by providing a layer of a hard, low friction machinable material over the mating surface of the compression piston. One such method applies a composite layer of bearing material in the form of a flexible tape bonded onto the mating surface of the piston. The flexible tape may include a polymetric reinforced layer of polytetrafluoroethylene (PTFE), however, other PTFE based composite materials may also be used. One such material is available under the trade name RULON J from DIXON DIVISION OF FURON of Bristol, R.I., USA. It is known in the art to bond a layer of RULON J tape to the piston mating surface.
RULON J as well as other PTFE based composite layers may be machined or ground after bonding onto the piston mating surface. In such applications, it is recommended to finish a mating cylinder wall with a relatively rough surface finish, e.g. 16 micro inches Ra, and then to wear in the PTFE based bearing material layer bonded to the piston mating surface by installing the piston into the mating cylinder and by cycling the piston over many hundreds or thousands of cycles. The mating pair is then disassembled, cleaned and reassembled for final manufacture. This process allows portions of the PTFE composite layer of bearing material bonded to the piston to penetrate the relatively rough cylinder wall thereby depositing a portion of the friction reducing layer into and onto the cylinder wall while at the same time smoothing the cylinder wall to a final surface finish during the wear in cycle. The wear in process although effective is undesirable since it adds time and labor to the overall manufacturing process. This process also reduces the overall life of the compressor since the wear-in process actually increases the clearance between the piston and the cylinder wall before the compressor is actually in use, thereby reducing its useful life.
It is therefore a general problem in the art to reduce the radial clearance between a cryocooler compression piston and its mating compression cylinder wall.
It is a further problem to manufacture cryocooler compression piston and compression cylinder elements with a high geometric accuracy for providing a more uniform radial clearance or circumferencial gap between the piston and cylinder wall mating surfaces.
It is a still further problem to reduce friction between a cryocooler compression piston and its mating cylinder wall so that compressor drive input power and heat generation are reduced.
It is still further problem to manufacture cryocooler compression pistons and cylinders from materials having a higher thermal conductivity than steel thereby more readily removing thermal energy from the compressor elements.